feat(i18n): full RTL/CJK support — fonts, Japanese locale, directional fixes

Bundle Noto Sans Arabic + Noto Sans JP (SIL OFL 1.1) as per-glyph fallbacks
for both the UI text theme and every generated PDF, so RTL (Arabic) and CJK
render on all platforms — including desktop, where the system font may lack
them — instead of tofu. A shared pdf_fonts.dart centralizes the PDF theme and
the three label/catalog/recovery services reuse it.

Add Japanese (ja) as the CJK reference locale — fittingly, the app's own name
is Japanese (種, tane, 'seed'). Core strings translated; the rest fall back to
English via slang. Wired into supportedLocales (automatic) and the settings
picker.

Fix three physical Alignment.centerLeft -> AlignmentDirectional.centerStart in
variety_detail_screen, and make month-abbreviation and avatar-initial
truncation grapheme-safe (.characters) so CJK text is never cut mid-character.

Tests: CJK/Arabic PDF glyph render, ja locale resolution + English fallback.
The chat-bubble mirroring flagged by the audit was verified correct (already
respects Directionality) and left unchanged.

apps/app_seeds/lib/ui/theme.dart

@@ -52,6 +52,11 @@ ThemeData buildTaneTheme() {
   return ThemeData(
     useMaterial3: true,
     colorScheme: scheme,
+    // Per-glyph script fallbacks so Arabic (RTL) and CJK text render even where
+    // the platform's default font lacks them (notably desktop). The primary
+    // family stays the system default for Latin; these only fill the gaps. The
+    // ranges are disjoint, so order is immaterial. Fonts bundled in pubspec.
+    fontFamilyFallback: const ['Noto Sans JP', 'Noto Sans Arabic'],
     scaffoldBackgroundColor: seedCanvas,
     // Green app bars — aligned with the v2 mockups and the real app.
     appBarTheme: const AppBarTheme(